ClearCase: Enable Diff, History and Annotate only for elements

Diff, History and Annotate only makes sense for ClearCase elements.

Change-Id: I81f6f5d3fcc81d7c493cc6b3a621902fb57cabe4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
ClearCase: Better debug output

Change-Id: I1daeda0159558377a43f04845b974155ddc77d57
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
ClearCase: Check if ClearCase command is executable

Change-Id: I8e873fcb1bdc3543a031a55b8e027a574a101752
Reviewed-by: Orgad Shaneh <orgads@gmail.com>